1. Field of the Invention
The present invention relates to improvements and integration in a display control system.
2. Description of the Prior Art
Conventionally, as a display, a liquid crystal display (which is referred to as LCD hereinafter) was smaller in display capacity when compared with a cathode ray tube (which is referred to as CRT hereinafter). For example, CRT could display characters of 80 columns (horizontally) by 25 rows (vertically) per picture, while LCD could display characters of 40 columns by 4 rows only per picture. For this reason, a display control system for LCD was quite different from a display control system for CRT, and neither of the two display control systems could be used in common.
Recently, a hand-held computer (which is referred to as HHC) has been widely applied, and, because of the need of low power consumption, an LCD display system is used for an HHC display system. In this case, LCD is divided in the vertical direction of the screen into two sections, which are positioned closely to each other to provide a parallel display. Also, HHC is highly advancing in performance and function, with the display performance of LCD approaching to that of CRT. Therefore, it is now desired to develop a display control unit or system which is capable of driving both of LCD and CRT display units in the HHC field. The reason why such desire is expressed lies in the recognition that LCD and CRT can be essentially controlled in display in the same manner.
Actually, however, there are some problems to be solved. First, because the display specifications of LCD are fixed physically (for example, the number of the horizontal scanning lines is fixed according to LCDs) and LCD requires no vertical or horizontal retrace lines, such a sufficient display control as in CRT can not be carried out for LCDs having various specifications while the comapatibility of software is perfectly maintained.
Secondly, conventionally, since LCD is unable to provide a gradation or tone display (half tone display), it cannot give the same display as CRT.
Thirdly, because LCD is generally slow in display clock speed, the associated display control system is accordingly slow in speed. This causes slow access from memory, and thus in the LCD display the system performance is reduced when compared with the CRT display.
Fourthly, in the conventional display control systems, external registers can not be added to them. Therefore, the number of necessary circuits must be increased to perform a configuration control when it is necessary.
Fifthly, when the conventional software is used, if the system is expanded in function, the function-expanded portion of the system can not be protected in execution of the software.